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CO2 /CH4 Separation with Ionic Liquids

Cristina Isabel Fernandes FerreiraOrientador: Professor Doutor João Araújo Pereira Coutinho


Summary

1. Introduction

Environmental Issues-Climate Change

CO2 Mitigation

CO2/CH4 separation

Ionic Liquids

Ionic liquid properties

2. Results and Discussion CO2 solubility relation with IL surface tension and molar volume

Non ideality of CO2 /non volatile solvents systems

Non ideality of CH4/ILs systems

CO2 /CH4 Selectivity

Non ideality of CO2 /protic ionic liquid systems

3. Conclusions


1. IntroductionEnvironmental Issues- Climate Change

CO2 accounts for about 80 % of the enhanced global warming effect

Increasing atmospheric concentration of several greenhouse gases (GHGs),

caused by human activities

Main sources of CO2

CO2 emissions must be reduced


CCS consists of :

1. Capturing CO2 from industrial or powerplants;

2. Compressing and transporting it;3. Storing it in geological formations by

injecting it into suitable, permanent sites underground.

Strategies to prevent CO2 emission

o Energy efficiency improvements

o Less carbon intensive fuels

o Nuclear power

o Renewable energy sources

o Enhancement of biological sinks

Techniques aiming at the sequestration of CO2 (CCS)

CO2 mitigationOptions

1. IntroductionCO2 Mitigation


1. Introduction Why separate CO2 from CH4 ?

The separation of CO2 from CH4 is an important process in many industrial areas :• Natural gas processing • Biogas purification • Enhanced oil recovery • Flue gas treatment

Natural gas:

• Novel transport technologies• The remarkable reserves found• The lower overall costs • The environmental sustainability

• Less polluting than oil and coal• Used in more efficient plants

Carbon dioxide reduces the heating value

Takes up volume in the pipeline

Causes corrosion in pipes and process equipment (acidic gas)

Atmospheric pollution from the combustion of natural gas

CO2 content in natural gas varies from 4 to 50% and must be reduced to 2–5%

Primary energy source in the near future


1. IntroductionGas treating Technologies


1. IntroductionIonic Liquids

Consist entirely of ionic species

Fluid at room temperature

Cation is generally a bulk organicstructure with low symmetry

Anion is mostly inorganic

They can be used in gas separationprocesses as solvents, replacing thetraditional solvents.

Choices of cations and anions are numerous


1. IntroductionIonic Liquids – General Properties

Present a low melting point (<100ºC), remaining liquid at Room temperature

Very wide temperature range in the liquid state

Non flammable

Non explosive

Good electrical conductivities

Low vapor pressure

Thermally stable

Large electrochemical stability window

They can be designed to posses a particular set of properties ( easily modified structure)

Designer Solvents


The major drawbacks of the traditional gas absorption separation processes are mainly caused by the nature of the solvent, and the type of interactions given between the solute and the solvent.

Ionic Liquids have been proposed as promising candidates for CO2

absorption and separation.

To design and optimize processes using ILs, it is essential that we understand the important factors that determine CO2 solubility.

As it is possible to design many potential ILs, the study of them will allow us to discover the key properties in making CO2 -philic ILs.

1. Introduction


2. Results and DiscussionCorrelation of solubility with IL surface tension

In general there is a relation between the solubility and the surface tension of the solvent.

The solubility of CO2 and CH4 in ionic liquids decreases as the surface tension increases.

High scatter of the data is observed

No correlation can be obtained


The solubility of both gases is dependent on IL molar volume

With an increase in the molar volume of the solvent, the gas solubility increases

For very large molar volumes, an increase of this thermodynamic property has no longer a significant effect on solubility

The relation can be expressed by a potential equation

2. Results and DiscussionCorrelation of the solubility with IL molar volume

670.00241.02

VmHCO

827.16105036.14

VmHCH


2. Results and DiscussionNon ideality of CO2/non-volatile solvent systems

Ideal solution described by Raoult's law that is defined as

Understanding of the molecular mechanism that dominates the CO2 sorption

Solvents are non-volatile Vapor phase is pure CO2

VLE data was collected for systems of CO2 with Ionic Liquids, alcohols, alkanes, fatty acids and fatty acid esters compared with those on an ideal solution

2222... COCOCOCO pxpy

22. COCO pxp

•Design and operation of processes•Design of enhanced solvents for CO2

12coy

ideal solution: 1


CombEsEE GGG ,Re,

2. Results and DiscussionFlory-Huggins model

• Deviations to ideality of a given mixture can be quantified estimating its excess Gibbs Energy

Contribution due to thedifferences in size and

shape

Contribution due to theinteractions

• The non ideality can be described in terms of the activity coefficient, γ

)ln(ln

:,,

i

jinjPTi

E

T RTn

Gn

222.. COCOCO pxp


2. Results and DiscussionFlory-Huggins model

The fluid structure of the IL can be approximated by a solid-like structure

Flory Huggins Model

Contribution to non ideality causedby entropic effects

2

2

2

2

2

2.1lnexp. CO

CO

CO

CO

CO

CO pxx

xp

solventesolventeCOCO

COCO

COVxVx

Vx

..

.

22

22

2

Modified Raoult’s Law:


2. Results and DiscussionNon ideality of CO2/Alkane systems

The Flory-Huggins model predicts

negative deviations to ideality

The VLE data has near ideal

behavior

There is an effect of positive

deviations from the residual part

CO2-alkane interactions must be

weaker than the alkane-alkane

interactions

Solute – solvent interactions don’t

energetically compensate the

interactions that are destroyed

upon mixing


Strong CO2-OH interactions observed

spectroscopicallyStrong solute-solvent interactions

are not enough to guaranteeenhanced solubility

Non ideality of CO2/Alcohol systems

2. Results and Discussion

VLE data is placed above the ideality line

Positive deviations to ideality

Intermolecular forces between molecules of CO2 and alcohol are

less important than they are between alcohol-alcohol

molecules


2. Results and DiscussionNon-ideality of CO2/Fatty acid esters and CO2/IL systems

Deviations are intermediate between the ideal behavior and the predicted by the Flory-Huggins

model

Combinatorial term of the Gibbs free energy is larger than the residual

Solubility of CO2 in ILs is controlled by entropic effects

Large negative deviations to the ideal behavior

The residual contribution for the non ideality is negative

CO2carbonyl interactions are energetically favorable when compared with the carbonyl-

carbonyl interactions established between the ester molecules


0. ii mHP

Carvalho,P. J. and Coutinho,J.A.P, On the Nonideality of CO2 Solutions ion IonicLiquids and other low volatile solvents . The Journal of Physical Chemistry

Letters,2010(4),p.774-780

2. Results and Discussion

CO2 solubility is solvent independent

Solubility of CO2 in non volatile solvents expressed in molality follows the same

behavior

A linear relation was adjusted to the p-m diagram at different temperatures and

from the slope the Henry’s law constant was taken

converted to mole fraction basis (H1)


IL molar volume can be related to IL molecular weight by a linear regression

Relation of the CO2 solubilitywith IL molecular weight

2. Results and DiscussionRelation of the molar volume to molecular weight

670.0)9832.0(0241.02 rCO MH

670.00241.02

VmHCO

MrVm 9832.0

The prediction of CO2

solubility in IL based on IL molecular weight seems

to be a good approximation

Henry’s law constants

obtained in both ways at 303 K


2. Results and DiscussionSolubility dependence with temperature

CO2 solubility seems to be strongly temperature dependent

As the temperature increases, the solubility becomes lower

The isotherms are well separated The isotherms practically overlap each other

Low temperature dependence of CH4

solubility

The solubility remains almost the same for different temperatures

CO2 and CH4 solubility in the ionic liquid [C4mim][CH3SO4]


2. Results and DiscussionDeviations to ideality of CH4 Solubility in ILs

VLE data lies above the ideal line

Strong positive deviations to ideality

CH4-IL interactions are non favorable and much weaker than the IL-IL ones.

The solubility increases as the chain length on the imidazolium ring of the cation increases in the order [C2mim]

<[C6mim]

VLE data for CH4/IL systems are compared to the ideal behavior that is assumed to be given by the CH4/Hexane system.

The pyridinium cations present a higher solubility of CH4 relatively to the imidazolium cations

The solubility increases for the same cation in the order [PF6]< [BF4]< [CH3SO4]


2. Results and DiscussionCO2/CH4 Selectivity

From a CH4/CO2 mixture the component CO2 is absorbed preferentially

For all systems the selectivity is greater than the ideal and decreases as the temperature increases

The largest selectivities were observed at a temperature of 298K and for the ionic liquid [C1mim][CH3SO4] and [C2mim][BF4].

2

4

42

CO

CH

CHCOH

HS


2. Results and DiscussionCO2/CH4 Selectivity dependence with IL molar volume

The selectivity decreases with the ionic liquid molar volume

Low molar volume ionic liquids are ideal for the separation of a CO2 from CH4

The data available is however very limited and a define conclusion can only be achieved when more solubility data becomes available.


2. Results and DiscussionNon ideality of CO2/Protic ionic liquids systems

This family of ionic liquids is not a viable option for CO2 capture when compared with the other ionic liquids studied.

Solubility data from literature lie always above the ideal behavior line

Large positive deviations to ideality

A CO2-protic IL interaction is less favorable than a IL-IL one

Formation of the EDA complex is not favorable.


3. Conclusions

In general it can be concluded that the solubility of CO2 in non volatile solvents depends on the combination of the different interactions involved, namely the solute-solute, solvent-solvent and solute-solvent.

The favorable interactions between CO2 and the solvent are not the exclusively factor that influences the solubility.

CH4-IL interactions are much weaker than the IL-IL ones, since the CH4-IL systems have strongpositive deviations to ideality.

The solubility of CH4 becomes influenced by the alkyl chain length in the cation, representinghigher solubility for longer chain lengths on the imidazolium ring.

The greater the polarity of ILs the higher the deviation to ideality in the CH4-IL systems.

Prefered solubility of CO2 in ILs compared to that of CH4.


Based on the selectivity the most adequate solvents are [C1mim][CH3SO4] and [C2mim][BF4].

As the IL surface tension increases the solubility decreases

An increase in the molar volume implies an increase in the gas solubility, but as the molar volumebecome very large the effect becomes insignificant.

Solubility data of CO2 in ILs can be obtained knowing a parameter as simple as the molar mass ofthe solvent.

Selectivity decreases with the ionic liquid molar volume and thus low molar volume ionic liquidsare ideal for the separation of CO2 from CH4

Protic ionic liquids are not favorable as solvents for CO2

3. Conclusions


Thank you for yourAttention!

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